The vascular beds supplying the different organs of the body are structurally and functionally specialized, allowing an optimal matching of blood flow with their individual requirements.

Coronary Circulation

The anatomy of the coronary circulation is described in Chapter 2.

The high capillary density of the myocardium (∼1 capillary per muscle cell) allows it to extract an unusually large fraction (about 70%) of the oxygen from its blood supply. The resting blood flow to the heart is relatively high, and moreover increases approximately fivefold during strenuous exercise.

Figure 25a shows left and right coronary blood flow during the cardiac cycle at a resting heart rate (HR) of 70 beats/min. During systole, the branches of the left coronary artery that penetrate the myocardial wall to supply the subendocardium of the left ventricle are strongly compressed by the high pressure within the ventricle and its wall. Left coronary blood flow is therefore almost abolished during systole, so that 85% of flow occurs during diastole. Conversely, right coronary arterial flow rate is highest during systole, because the aortic pressure driving flow increases more during systole (from 80 to 120 mmHg) than the right ventricular pressure which opposes flow (from 0 to 25 mmHg).

With a HR of 70 beats/min, systole and diastole last 0.3 and 0.55 s, respectively. As the HR increases during exercise or excitement, however, the duration of diastole shortens more than that of systole. At 200 beats/min, for example, systole and diastole both last for 0.15 s. In order to cope with the greatly increased oxygen demand of the heart, which occurs simultaneously with a marked reduction in the time available for left coronary perfusion, the coronary arteries/arterioles dilate dramatically to allow for a pronounced rise in blood flow. The mechanism of this exercise hyperaemia in humans is unknown. However, in the porcine coronary circulation, which seems to be regulated in a manner similar to that of humans, coronary exercise hyperaemia is probably caused by reduction of endothelin release from the coronary endothelium, plus a sympathetically mediated β-receptor activation of coronary artery smooth muscle which results in a rise in cellular cyclic AMP (cAMP) and the opening of BK_Ca channels.

Cutaneous Circulation

Apart from supplying the relatively modest metabolic requirements of the skin, the main function of the cutaneous vasculature is thermoregulation, the maintenance of a constant body temperature. Vascular tone in the skin is regulated by both neural reflexes and local cutaneous mechanisms that respond to temperature. Temperature increases cause cutaneous vasodilatation, allowing more blood to flow to the skin and radiate its heat to the environment to promote body cooling; decreases in temperature have the opposite effect.

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